There have been known the electrophotographic system, the heat transfer recording system through sublimation and melting, and the ink-jet recording system, as the image-recording techniques in which images are recorded or formed on a recording medium such as a sheet of paper on the basis of image data signals. The electrophotographic system requires the use of processes for charging a light-sensitive drum and for exposing the same with light rays to thus form electrostatic latent images and accordingly, this technique suffers from problems in that the system to be used becomes quite complicated in its structure and this in turn leads to an increase in the production cost. On the other hand, the heat transfer recording system requires the use of a cheap device, but the system uses an ink ribbon and accordingly, problems arise such that the running cost is quite expensive and this technique results in the generation of a large quantity of waste materials.
On the other hand, the ink-jet recording system requires the use of a cheap device and can directly form images on a recording medium by discharging ink onto only the desired image-forming regions of the recording medium. Accordingly, this technique would permit the effective use of the ink and result in a cheap running cost. Moreover, this recording technique never generates so much noise and therefore, this system is quite excellent as the image-recording system.
Among the ink-jet recording systems, the radiant ray-curing ink-jet recording system in which the ink is cured through the irradiation thereof with activated radiant rays such as ultraviolet rays has recently attracted special interest because the system relatively slightly gives out bad smells and the ink has a quick drying ability and the ink permits the recording of images even on a recording medium free of any ink-absorbing ability. In particular, the UV-curing ink-jet recording system which makes use of a solvent free-ink composition without using any solvent has recently attracted special interest, from the viewpoint of the protection of the working environment and the control of discharging VOC (Volatile Organic Compound) as well as the viewpoint of the ability of forming images within a short period of time without passing through any additional step such as a drying step. In the ink-jet recording system, however, the ink should be discharged through a quite fine nozzle (having a diameter ranging from, for instance, 1 to 100 μm) and accordingly, it would be quite important that the ink composition used has a viscosity value falling within a predetermined range (for instance, a viscosity ranging from 5 to 50 cP at a discharge temperature of 25° C.). For this reason, the ink-jet recording system requires the use of an ink composition which never requires the use of any solvent (a solvent-free type ink composition) and which has an appropriate viscosity value at the same time.
In addition, the radiant ray-curing ink-jet recording system should be cured at a high sensitivity and would permit the formation of high quality images. In other words, it would be necessary in the radiant ray-curing ink-jet recording system that images should be irradiated with radiant rays within a very short time period (for instance, 0.01 to 0.5 seconds) after the impact of such an ink composition on a recording medium to thus cure the ink composition before the recording medium is never blurred with the ink, but this requires the use of an ink composition having a quite high sensitivity (very fast curing rate). Moreover, the ink composition is discharged from the ink-jet recording device in the form of droplets and therefore, the ink suffers from such problems that it is quite susceptible to the polymerization-inhibitory effect of oxygen and that the sensitivity thereof is in turn impaired. A high curing ability encountered when irradiated with activated radiant rays can be imparted to an ink composition through the improvement of the sensitivity thereof and the improvement of the sensitivity would likewise permit the achievement of other various advantages such as an increase of the life time thereof through the reduction of electric power consumption and the reduction of any load applied to the radiant ray-generator, and the control of the volatilization of any uncured low molecular weight substances and any reduction of, for instance, the strength of the resulting images. In addition, when applying the ink composition to the formation of image areas for a lithographic printing plate, the improvement of the image strength through the enhancement of the sensitivity of the ink composition would impart high printing durability to the resulting lithographic printing plate.
Accordingly, there has been desired for the development of a solvent-free type radiant ray-curable ink-jet ink composition, which has an appropriate sensitivity to radiant rays and likewise has a predetermined viscosity value.
There has been proposed a composition comprising a polymerizable compound selected from the group consisting of specific acrylate compounds (see, for instance, Patent Document 1 specified below) for the purpose of, for instance, providing a highly safe ink composition having the following various advantages: images can be formed even on supports (such as those made of polyester, polycarbonate, polypropylene or the like), whose direct recording by the ink-jet recording method is in general quite difficult, without being accompanied by any blurring of the support, at a high sensitivity, the ink composition is strongly adhered to a recording medium, and the composition is highly safe since it never causes any stimulation and any sensitization of the skin. However, the multi-functional acrylates used for the improvement of the sensitivity provide, as a result of curing, polymers each having a highly crosslinked three-dimensional structure and accordingly, the images obtained after the cure of the ink composition have insufficient flexibility and reduced impact resistance, this in turn leads to the formation of images quite susceptible to the formation of cracks and peeling off of images and accordingly, the resulting images shows insufficient durability.
Moreover, in the conventional techniques for the preparation of a lithographic printing plate, there has been adopted a method which makes use of a so-called PS plate comprising a hydrophilic substrate provided thereon with a lipophilic light-sensitive resin-containing layer and which comprises the steps of imagewise exposing the resin layer constituting the light-sensitive layer to improve or reduce the solubility of the exposed area thereof in an alkaline developer and to thus form desired images and then removing the non-image area through the dissolution thereof in the developer. However, there has recently been widely used a digital technique in which image information is electronically processed, stored and outputted using a computer, and accordingly, there has correspondingly been desired for the development of a novel image-outputting system. In particular, various attempts have been made to develop a method which can prepare a printing plate without passing it through any treatment with a developer and to develop a method for directly preparing a lithographic printing plate while using an ink-jet ink composition (see, for instance, Patent Document 2 specified below). Such a method comprises the steps of imagewise discharging an ink composition onto the surface of, for instance, a hydrophilic substrate according to, for instance, the ink-jet recording technique and then curing the ink composition through the irradiation thereof with activated radiant rays to thus give a printing plate provided thereon with a desired pattern of images (preferably hydrophobic images). To form practically acceptable image areas of a lithographic printing plate, the ink composition to be used should satisfy the following requirements: the droplets of the ink composition discharged onto the surface of a substrate can immediately be cured without being accompanied by any blurring; it can ensure the formation of cured images excellent in the strength and the adhesion to the substrate; and when fitting the resulting lithographic printing plate to a printing press, the image area can appropriately follow-up with the deflection of the substrate and never causes any defect such as a crack. Thus, there has presently been desired for the development of such an ink composition suitably used in these applications.
Patent Document 1: Japanese Un-Examined Patent Publication 2003-192943;
Patent Document 2: Japanese Un-Examined Patent Publication Sho 54-117203.